Webbed foot
Different vertebrate species with webbed feet have different mutations that disrupt this process, indicating that the structure arose independently in these lineages.
These conditions also demonstrate a variety of genetic targets for mutation resulting in webbed feet, which may explain how this homologous structure could have arisen many times over the course of evolutionary history.
In experiments with chickens, mutations to a BMP receptor disrupted the apoptosis of interdigital tissue and caused webbed feet similar to ducks to develop.
[11] These results indicate that in avian lineages, the disruption of BMP signaling in interdigital tissue caused webbed feet to arise.
[13] This demonstrates that webbed feet arise from developmental changes, but do not necessarily correlate with a selective advantage functionally.
Most webbed-footed species spend part of their time in aquatic environments, indicating that this homologous structure provides some advantage to swimmers.
Of the three orders of amphibians, Anura (frogs and toads) and Urodela (salamanders) have representative species with webbed feet.
Salamanders in arboreal and cave environments also have webbed feet, but in most species, this morphological change does not likely have a functional advantage.
Most webbed-footed animals utilize paddling modes of locomotion where their feet stroke backwards relative to their whole body motion, generating a propulsive force.
However, some waterfowl also utilize lift-based modes of propulsion, where their feet generate hydrodynamic lift due to the angle of attack of the foot and the relative water velocity.
For example, great-crested grebes use solely lift-based propulsion due to their lateral foot stroke and asymmetric, lobated toes.
Fully aquatic mammals and animals typically have flippers instead of webbed feet, which are a more heavily specialized and modified limb.
[27] However, for semiaquatic animals that mainly swim at the surface, webbed feet are highly functional; they trade-off effectively between efficient terrestrial and aquatic locomotion.
[29] In ducks, webbed feet have also enabled extreme forms of propulsion that are used for escape behaviors and courtship display.
Surface swimmers are speed-limited due to increasing drag as they approach a physically defined hull speed, which is determined by their body length.
Western and Clark's grebes utilize their lobated feet to generate nearly 50% of the force required to allow them to walk on water in elaborate sexual displays; they are likely the largest animal to "walk" on water, and are an order of magnitude heavier than the well-known lizards that exhibit a similar behavior.
[31] While webbed feet have mainly arisen in swimming species, they can also aid in terrestrial locomotors by increasing contact area on slick or soft surfaces.
[32] However, some ecologists believe that their webbed feet do not aid aboveground locomotion, but are mainly utilized as shovels for burrowing and digging in the sand.
